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1.
桐柏-大别造山带高压变质单元岩石Pb同位素组成   总被引：1，自引：1，他引：0  
铅同位素组成对于研究构造分区与演化、块体相互作用以及识别地壳中不同块体的上、下层次关系等具有重要意义.桐柏-大别造山带高压变质单元岩石的全岩Pb同位素组成研究表明, 在该造山带不同区段, 高压变质岩系二云钠长片麻岩与榴辉岩具有相似的Pb同位素组成, 表现为上部地壳高放射成因的Pb同位素组成特征, 其中Pb同位素组成为: 206Pb/204Pb=17.599~18.310, 207Pb/204Pb=15.318~15.615, 208Pb/204Pb=37.968~39.143.大别和桐柏地区高压变质岩系Pb同位素组成的一致性进一步证明了大别地区与桐柏地区的高压变质岩系是可以相连的, 它们应属于同一构造单元.高压变质岩系Pb同位素比值总体高于超高压变质岩系, 验证了桐柏-大别造山带扬子俯冲陆壳从下部岩系到上部岩系Pb同位素比值呈规律增长这一Pb同位素化学特征.侵入于高压变质岩系中的面理化(含榴)花岗岩, 其Pb同位素组成与高压变质岩系相比相对较低, 而与超高压变质岩系及其中的面理化(含榴)花岗岩相似, 为: 206Pb/204Pb=17.128~17.434, 207Pb/204Pb=15.313~15.422, 208Pb/204Pb=37.631~38.122.这表明高压变质岩系和超高压变质岩系中的面理化(含榴)花岗岩具有相同的岩浆来源.结合面理化(含榴)花岗岩具有A型花岗岩的地球化学特征分析, 它们的岩浆物质可能来自超高压变质岩折返至中下地壳的减压退变和部分熔融.   相似文献   

2.
There is a correlation between thorium and the light rare earth elements, indicated by La/Th ratios in fine grained sedimentary rocks of various ages from Australia and Greenland. The correlation between Th and the heavy rare earth elements (Th/Yb) is much less significant. Archean sedimentary rocks have a higher La/Th (3.6 ± 0.4) than post-Archean sedimentary rocks (La/Th = 2.7 ± 0.2).The cause of this correlation can be attributed to the coherent behaviour of these elements during most sedimentary processes (weathering, transport, diagenesis, etc.). Since the chondrite-normalized rare earth element distribution of clastic fine grained sedimentary rocks is accepted to be parallel to the distribution of REE in the upper continental crust, an estimate of upper crustal Th abundances can be made. Using reasonable assumptions of certain elemental ratios (K/U, Th/U, K/Rb) in the upper crust, minimum estimates of the abundances of K, U and Rb can also be made for the post-Archean and Archean upper crusts.The post-Archean values (K = 2.9%; Rb = 115 ppm; Th = 11.1 ppm; U = 2.9 ppm) compare favourably to some previous estimates made from direct sampling and theoretical considerations and help confirm a granodiorite present day upper continental crust. The Archean data (K = 0.92%; Rb = 30ppm; Th = 3.5 ppm; U = 0.92 ppm) support models which suggest a significantly more mafic exposed crust at that time.  相似文献   

3.
A total of 108 samples of meta-tonalites, metagranodiorites, granites and meta-tholeiites representing groups of Early to Late Archean age and different metamorphic history from SW and SE Greenland have been analyzed for Ca, K and 28 trace elements. There is no systematic change of the chemical composition with age observable. The results support petrologic experiments which suggest that tonalites and granodiorites (the most abundant rocks of the Archean crust) are partial melting products of a mafic lower crust. Modelling suggest that this crust consisted of garnet amphibolite derived from a source with a bulk composition resembling a slightly enriched rather than depleted mantle. The CeN/YbN ratio is above 10 in the majority of tonalites. Most samples have no Eu anomaly because of a balanced contribution from the minerals of a mafic rock (or a plagioclase-free source). The positive Eu anomaly of some granodiorites and of a minor proportion of tonalites can be explained as being caused by plagioclase accumulation during differentiation or by partial melting of plagioclase-rich fractions. Modelling with Zn excludes an origin of tonalitic melts by differentiation of basaltic to dioritic magmas. The Archean meta-diorites, meta-tonalites and meta-granodiorites from Greenland have generally lost some K and S relative to their suggested magmatic protoliths. Loss of Rb, Tl, Pb and K and relative gain of Ca, Sr, Ba and Sc connected with granulitization of meta-tonalites can be explained in the majority of cases by separation of about 25 percent granitic partial melt. High K/Rb, K/Pb, Zn/Cd and Nb/Th ratios of granulites plus low ratios of granites are almost in balance with intermediate ratios of amphibolite-facies tonalites. Retrogression of granulites into amphibolites was accompanied by introduction of Pb, Tl, Rb, Ba, Sr and K from Na-Cl-rich brines circulating on fractures. A comparison of the abundance of 24 elements (characterized by different compatibility) in the Archean crust of Greenland with the present bulk crust reflects only minor changes (Th, Nb) if at-all in the chemical composition of the continental crust since the Archean.  相似文献   

4.
Whole-rock Pb isotopic compositions of the high-pressure (HP) metamorphic rocks, consisting of two-mica albite gneisses and eclogites, and foliated granites from the HP metamorphic unit of the Tongbai-Dabie orogenic belt are firstly reported in this paper. The results show that the tip metamorphic rocks in different parts of this orogenic belt have similar Pb isotopic compositions. The twomica albite gneisses have ^206 pb/^204 Pb=17. 657 -18. 168, ^207pb/^204 Pb=15. 318-15. 573,^ 208Pb/^204ob=38.315-38. 990, and the eclogites have ^206Pb/^204 Pb=17. 599 -18. 310, ^207Pb/^204 Pb=15. 465 -15. 615,^208Pb/^204Pb=37. 968-39. 143. The HP metamorphic rocks are characterized by upper crustal Pb isotopic composition. Although the Pb isotopic composition of the HP metamorphic rocks partly overlaps that of the ultrahigh-pressure (UHP) metamorphic rocks, as a whole, the former is higher than the latter. The high radiogenic Pb isotopic composition for the HP metamorphic rocks confirms that the subducted Yangtze continental crust in the Tongbai-Dabie orogenic belt has the chemical structure of increasing radiogenic Pb isotopic composition from lower crust to upper crust. The foliated granites, intruded in the HP metamorphic rocks post the HP/UHP metamorphism, have ^206Pb/^204 Pb=17. 128- 17. 434,^207Pb/^204pb=15. 313-15. 422 and ^208Pb/^204Pb=37. 631-38. 122, which are obviously different from the Pb isotopic compositions of the HP metamorphic rocks but similar to those of the UHP metamorphic rocks and the foliated garnet-bearing granites in the UHP unit. This shows that the foliated granites from the HP and UHP units have common magma source. Combined with the foliated granites having the geochemical characteristics of A-type granites, it is suggested that the magma for the foliated granites in the UHP and HP unit would be derived from the partial melting of the retrometamorphosed UHP metamorphic rocks exhumed into middle to lower crust, and partial magmas were intruded into the HP unit.  相似文献   

5.
Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.  相似文献   

6.
Upper crustal abundances of trace elements: A revision and update   总被引：7，自引：0，他引：7  
Zhaochu Hu  Shan Gao   《Chemical Geology》2008,253(3-4):205-221
We report new estimates of abundances of rarely analyzed elements (As, B, Be, Bi, Cd, Ge, In, Mo, Sb, Sn, Te, Tl, W) in the upper continental crust based on precise ICP-MS analyses of well-characterized upper crustal samples (shales, pelites, loess, graywackes, granitoids and their composites) from Australia, China, Europe, New Zealand and North American. Obtaining a better understanding of the upper crustal abundance and associated uncertainties of these elements is important in placing better constraints on bulk crust composition and, from that, whole Earth models of element cycling and crust generation. We also present revised abundance estimates of some more commonly analyzed trace elements (Li, Cr, Ni, and Tm) that vary by > 20% compared to previous estimates. The new estimates are mainly based on significant (r2 > 0.6) inter-element correlations observed in clastic sediments and sedimentary rocks, which yield upper continental crust elemental ratios that are used in conjunction with well-determined abundances for certain key elements to place constraints on the concentrations of the rarely analyzed elements. Using the well-established upper crustal abundances of La (31 ppm), Th (10.5 ppm), Al2O3 (15.40%), K2O (2.80%) and Fe2O3 (5.92%), these ratios lead to revised upper crustal abundances of B = 47 ppm, Bi = 0.23 ppm, Cr = 73 ppm, Li = 41 ppm, Ni = 34 ppm, Sb = 0.075, Te = 0.027 ppm, Tl = 0.53 ppm and W = 1.4 ppm. No significant correlations exist between Mo and Cd and other elements in the clastic sediments and sedimentary rocks, probably due to their enrichment in organic carbon. We thus calculate abundances of these elements by assuming the upper continental crust consists of 65% granitoid rocks plus 35% clastic sedimentary rocks. The validity of this approach is supported by the similarity of SiO2, Al2O3, La and Th abundances calculated in this way with their upper crustal abundances given in Rudnick and Gao [Rudnick, R., Gao, S., 2003. Composition of the continental crust. In: Rudnick, R.L. (Ed.), The Crust. In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry, vol. 3. Elsevier–Pergamon, Oxford, pp. 1–64.]. The upper crustal abundances thus obtained are Mo = 0.6 ppm and Cd = 0.06 ppm. Our data also suggest a  20% increase of the Tm, Yb and Lu abundances reported in Rudnick and Gao [Rudnick, R., Gao, S., 2003. Composition of the continental crust. In: Rudnick, R.L. (Ed.), The Crust. In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry, vol. 3. Elsevier–Pergamon, Oxford, pp. 1–64.].  相似文献   

7.
The metamorphism and geochemistry of the major components of a small area of granulite facies rock are described and discussed, and a chemical model for the evolution of anomalous trace element distributions in such materials is suggested. The local complex was subjected to medium to high pressure granulite facies metamorphism between 2,900 and 2,600 m.y. All the analysed granulite facies rocks from Tiree; acid to intermediate gneisses, basic metamorphic rocks, and granitic rocks, have anomalous chemistries, being depleted in K, Rb, Nb, Y and Th, and have high K/Rb, Ba/Rb and Ca/Y ratios, and very low K/Ba and Rb/Sr ratios relative to normal portions of the upper continental crust. The gneisses seem to have been enriched in Ba and Sr.The chemical features of the rocks are considered to reflect their stable mineral assemblages in the granulite facies, and to be representative of deep-level crustal materials. The geochemical peculiarities of the complex may have been largely controlled by an upward intergranular diffusion, or “degassing” caused by high-grade metamorphism. It is suggested that such diffusion may have been active at the crust/upper mantle interface, some diffused material of mantle origin accounting for certain chemical oddities typical of Lewisian and some other Precambrian granulite facies rocks.  相似文献   

8.
Mass spectrometric analyses for rare earth elements (REE) have been carried out on some Precambrian mica schists, gneisses and granites from the Precambrian Numedal area, Norway and on their phyllosilicates. The rocks, which are metamorphosed in the upper greenschist to amphibolite facies, were originally partly sedimentary, partly magmatic.The total REE contents for rocks varies from 145 to 761 ppm. An average of 16 phyllosilicate samples gave 417 ppm REE (max. of 1809 ppm, min. of ca. 50 ppm). Coexisting light and dark phyllosilicates have similar abundances of REE. For the micas of high REE content most of the REE was extractable by rinsing with EDTA. The data thus support the possibility of an extensive adsorption of REE ions on micaceous minerals. The REE distribution patterns do not provide a clear distinction between the sedimentary and magmatic origin for the rocks examined.  相似文献   

9.
A Precambrian granulite terrain of the Brazilian shield presents an average Tl content of 350 ppb, very near to Shaw's figure for the lower continental crust. Thallium, as in other rock types, shows close coherence with Rb and K. The mean Rb/Tl value (193 for 53 samples) falls in the range of the ‘normal’ crustal values, while the K/Tl values (145,000 average) are higher than values for igneous rocks, as are the values for K/Rb. The results support the authors' hypothesis that granulitic rocks have undergone partial melting with contemporaneous depletion of Rb and Tl in relation to K.  相似文献   

10.
The granitic gneisses from the ultrahigh-pressure (UHP) metamorphic terrain of the southeastern Dabie Mountains encompass two types: monzonitic granitic gneiss and alkali-feldspar granitic gneiss, which are characterized by rich alkalis, poor CaO, high FeO/MgO, particularly high Ba, Rb, Th, Ta, REE (except Eu), Ga, Nb and Zn, and low Sr, Eu, Cr, Co and Ni. The gneisses, particularly the alkali-feldspar granitic gneiss, have typical chemical characteristics of A-type granites. They resulted from partial melting of crustal materials existing in the rift zone along the northern margin of the South China block during the Neoproterozoic. These gneisses might not have undergone UHP metamorphism during the late Triassic, but were involved into UHP rocks by the tectonic mixing process and kept the exhumation message of the UHP rocks from the middle and upper crust.  相似文献   

11.
大陆俯冲过程中的流体   总被引：5，自引：1，他引：5  
李曙光  侯振辉 《地学前缘》2001,8(3):123-129
含水矿物矿物稳定性的实验研究和超高压岩石的同位素地球化学研究表明 ,大陆地壳在俯冲过程中 ,随着变质程度的升高和部分含水矿物的相继分解 ,会有流体释放出来。当俯冲深度接近5 0km ,俯冲陆壳岩石中大量低级变质含水矿物 (如绿泥石、绿帘石、阳起石 )会脱水并从俯冲陆壳逸出形成流体流。这一流体流可溶解带走俯冲陆壳内已从云母类矿物逸出的放射成因Ar及部分U、Pb ,并导致w(U) /w(Pb)升高。这一阶段逸出的流体有可能交代、水化仰冲壳楔 ,为其发生部分熔融形成同碰撞花岗岩或加速山根下地壳的榴辉岩化创造条件。在俯冲深度为 5 0～ 10 0km ,变镁铁质岩石中的角闪石相继分解并释放出H2 O。由于变镁铁质岩石在陆壳中所占比例较少 ,因此 ,这一阶段释放的水不能形成大规模的流体流 ,因而不能使体系内的过剩Ar大量散失 ,但足以形成局部循环 ,加速变镁铁质岩石及其互层或邻近围岩的榴辉岩化变质反应。在俯冲深度 >10 0km的超高压变质阶段 ,仅有少量的含水矿物分解 ,而多硅白云母仍保持稳定。这时俯冲陆壳内只可能有少量粒间水存在 ,从而导致俯冲陆壳与周围软流圈地幔不能发生充分的相互作用。  相似文献   

12.
New trace element analyses have been made on the composite Canadian Precambrian shield samples reported in 1967.The overall mean abundance of Cr has been revised to 35 ppm (from 99). New abundances similar to 1967 values are (in ppm): Ni, 19; Co, 12; Cu, 14; Zr, 300; Sr, 315; Ba, 1070; Rb, 110: individual 1967 Rb values were erroneous.Elements not previously determined have the following overall mean values (in ppm): Zn, 52; Sc, 7.0; Nb, 26; Hf, 6.9; La, 32; Ce, 65; Nd, 26; Sm, 4.5; Eu, 0.94; Gd, 2.8; Tb, 0.48; Ho, 0.62; Yb, 1.5; Lu, 0.23; Y, 21; Pb, 17; values in ppb are: Ir, 0.02; Au, 1.8; Tl, 520.Clear positive correlations among Mg-Cr-Ni-Ir-Au appear for all rock-types, marble and quartzite as well as mafic igneous. Regional differences are apparent for several elements: e.g. higher Au, Ir, Cr, Ni in Baffin Island and Northern Quebec composites, compared with Saskatchewan and Southwestern Quebec; high Ti, Zn, Nb, Zr, Hf, REE, Y, Sr, K/T1 abundances and negative Eu anomalies in Southwestern Quebec.The overall REE abundances (omitting Southwestern Quebec) differ from other surface continental crustal rock estimates.  相似文献   

13.
The Ediacaran and early Cambrian black shales are widespread across the South China Craton (Yangtze and Cathaysia blocks). However, the trace element distribution patterns of the Ediacaran and early Cambrian black shales in the Cathaysia Block are still unclear. In this study, thirty-four black shale samples in the Lechangxia Group (Ediacaran) and thirteen black shale samples in the lower Bacun Group (early Cambrian) from Guangning area, western Guangdong Province, South China, were analyzed for major and trace elements concentrations. Compared to the upper continental crust, the Ediacaran black shales exhibit strongly enriched Se, Ga, and As with enrichment factor values (EF) higher than 10, significantly enriched Bi and Rb (10>EF>5), weakly enriched Mo, Ba, Cs, V, In, Be, Tl, and Th (5>EF>2), normal U, Cr, Cd, Sc, Pb, Cu, and Li (2>EF>0.5), and depleted Ni, Zn, Sr, and Co. Early Cambrian black shales display strongly enriched Se, Ga, and As, significantly enriched Ba, Bi, and Rb, weakly enriched Mo, Cs, Cd, V, U, Be, In, and Tl, normal Sc, Th, Cr, Li, Cu, Ni, and Pb and depleted Co, Zn, and Sr. Moreover, Se is the most enriched trace element in the Ediacaran and early Cambrian black shales: concentrations vary from 0.25 to 30.09 ppm and 0.54 to 5.01 ppm, and averaging 4.84 and 1.72 ppm, with average EF values of 96.87 and 34.32, for the Ediacaran and early Cambrian shales respectively. The average concentration of Se in the Ediacaran black shales is 2.8 times higher than that of early Cambrian black shales. Se contents in the Ediacaran and early Cambrian black shales exhibit significant variation (P = 0.03). Provenance analysis showed that Se contents of both the Ediacaran and early Cambrian black shales were without detrital provenance and volcanoclastic sources, but of hydrothermal origin. The deep sources of Se and the presence of pyrite may explain the higher Se contents in the Ediacaran black shales. Similar with the Se-rich characteristics of the contemporaneous black shales in the south Qingling Mountain and Yangtze block, the Ediacaran and early Cambrian black shales in Guangning area, Cathaysia, are also enriched in Se, which may provide a clue for looking for the selenium-rich resources in western Guangdong Province.  相似文献   

14.
Proterozoic basement outcrops in the vicinity of Battye Glacier, northern Prince Charles Mountains, are dominated by granulites and gneisses derived from felsic (granitoid) intrusive igneous rocks, and by pegmatites. Felsic orthopyroxene granulites, garnet leucogneisses and garnet pegmatites have major and trace element compositions of highly felsic, but not strongly fractionated, granites. The garnet leucogneisses and garnet pegmatites have S‐type characteristics, whereas the felsic granulites are probably I‐type, although their high Zr+Nb+Y+Ce abundances suggest possible A‐type affinities. Intermediate orthopyroxene ± clinopyroxene granulites mostly resemble I‐type quartz diorites, except for a small subgroup of samples (characterised by low Na2O and K2O, and high MgO, Ni, Cr and HREE) of uncertain affinities and significance. Element ratios involving LILE (e.g. K/Rb, Rb/Ba, Rb/Sr, K/La, La/Th) closely match those typical of the inferred granitoid protoliths, suggesting that these rocks have experienced relatively little LILE depletion (except possibly for U) during regional metamorphism. It is therefore inferred that metamorphism was probably broadly isochemical. Because the felsic and intermediate granulites and garnet leucogneisses are Sr‐depleted, Y‐undepleted and mostly have negative Eu anomalies they are inferred to be the products of partial melting of felsic crustal sources leaving plagioclase‐bearing residua. Plagioclase fractionation during crystallisation could also account for these characteristics, but K/Rb, Rb/Ba and Rb/Sr ratios in these rocks are not consistent with strong fractionation of feldspar. Garnet pegmatites differ chemically from garnet leucogneisses mainly in their lower Fe, Ti, Nb, Zn, Zr, Th and REE abundances and positive Eu anomalies, related to lower garnet, ilmenite and zircon contents in the garnet pegmatites. A genetic link between these two rock types, probably involving fractionation of these minerals during partial melting or crystallisation, is inferred. Incompatible‐element abundances suggest that generation of the Battye Glacier granitic magmas from felsic crust might have occurred in a mature continental magmatic arc possibly well removed from an active subduction trench or, perhaps, in an intracontinental setting.  相似文献   

15.
山东招远前孙家金矿床地质和元素地球化学研究   总被引：6，自引：1，他引：5  
对矿区地表和井下四个中段共采集地层、岩体、蚀变岩、石英脉和矿体样品503件,测试了Au、Ag、As、Sb、Cu、Pb、Zn、Sn、Bi、Mo、Co、Ni、Mn、Cr、V、Ti、Ba、Rb和Sr等19种元素的含量,运用Suffer软件编制了19种元素的矿脉地球化学纵剖面图,根据格里戈良分带指数法计算了元素的地球化学分带,建立了原生晕地球化学立体分带模式。综合地质地球化学资料分析得出以下认识：由花岗岩→钾长石化花岗岩→绢英岩化花岗岩→绢英岩→石英脉或硅化带→金矿石,随着蚀变作用增强,成矿元素Au、Ag、As、Cu、Pb、Bi、Mo、Sb增加,然而成矿作用主要发生在绢英岩化后的硅化阶段。矿区成矿元素Au、Ag、As、Cu、Pb、Sn、Bi、Mo、Co．Ni含量呈多峰分布,分布范围大,离散度大,其中Au、Ag高含量峰值区峰型突出,成矿富集趋势强烈。矿区成矿阶段可分为：石英-金-黄铁矿阶段、石英-金-多金属硫化物阶段和石英-碳酸盐阶段。F1因子（Au、Ag、As、Co、Ni、Sn、Bi）为主成矿阶段的元素组合,F3因子（Mo、Sb）和F5因子（Cu、Zn）反映了成矿热液活动的多期叠加。F4因子Rb、Sr、Ba组合在本区最具典型意义,Rb与Sr、Ba为对抗性元素,Sr、Ba、Ca低值带,Rb、K高值带恰好与Au、矩高值带吻合,显示花岗岩长石类矿物的钾长石化和绢云母化与成矿关系密切。与矿体的侧伏方向一致,总体上矿脉纵剖面地球化学高值带均向SW侧伏。其中Au、Ag高值带宽且比较连续,上下均未封闭,结合金矿体虽已出露地表但规模不大和原生晕分带特征,推测为一浅剥蚀矿床,向SW深部仍有一定延深。矿区原生晕分带序列自上而下为（Hg2、Sr、Ba、Rb）→（矩、Au、Sn、Mo、Cu→（Zn、Pb）→（Sb、Mn）→（Ni、V、Co、As、Hg1、Bi）。As、Sb、Hg高值带偏于中下部,可能指示下部隐伏矿体的存在。  相似文献   

16.
川西同德和沙坝麻粒岩及其退变质岩石之间的元素迁移   总被引：6，自引：0，他引：6  
利用川西麻粒岩的全岩化学组成与岩石中特征矿物的化学性质,研究了麻粒岩退变质作用的元素迁移。对紫苏辉石、普通辉石和普通角闪石的电子探针分析表明,随着麻粒岩退变质作用程度的加深,矿物的化学组成与全岩的元素迁移同步变化。由于矿物组合和岩相的转变以及地壳流体的作用,同德二辉麻粒岩在退变质至黑云角闪斜长片麻岩过程中,K,Rb,Cs,K／Na,Fe^3+和Fe3+／Fe^2+增加,Fe^2+,V,Co,Ba,Sr和Pb降低;在沙坝二辉麻粒岩退变质为含绿帘石角闪斜长片麻岩过程中,Na,Ca,Fe^3+,Zr,Hf,Nb,Ta,Rb,U,Th,REE和Fe^3+／Fe^2+增加,Fe^2+,Mg,K,Co,Zn,Cs,Pb和K／Na降低。麻粒岩退变质过程中制约不同元素迁移的主要矿物相是普通角闪石等退变质新生矿物。同德和沙坝两个麻粒岩块退变质作用中,个别元素迁移性状并不相同,这可能与原岩的性质和变质流体的成分等环境地质条件有关。  相似文献   

17.
赵善仁  吴悦斌 《现代地质》1996,10(4):478-484
五台山—恒山绿岩带Ａｕ、Ａｇ、Ｃｕ矿床可分为二大类型：（１）再生型金银铜矿，产在包括岩浆岩在内的各类岩石断裂构造中，与岩浆期后热液有关；（２）变生型金银铜矿，产于各类变质岩中，具有层控特征（即绿岩型金矿）。在地球化学特征上，再生型矿床与变生型矿床相比，矿体及围岩中Ｍｏ、Ａｇ、Ｐｂ、Ｚｎ、Ｃｄ等成矿及伴生元素明显富集；Ｋ２Ｏ、Ｒｂ、Ｓｒ、Ｂａ、Ｔｈ、Ｕ也明显富集，是后期岩浆热液作用的结果；Ｈｇ、Ｆ的明显富集则与后期构造活动有关；Ｚｎ／Ｃｄ比值较低，说明受到后期岩浆侵入影响；Ｔｈ／Ｕ比值低，可能指示富钙的酸性岩环境。再生型Ａｕ矿化的元素组合为Ｃｄ、Ａｓ、Ｎｉ、Ａｇ、Ｓｂ、Ａｕ、Ｈｇ（Ｂｉ），再生型Ａｇ矿化的元素组合为Ａｓ、Ｓｂ、Ａｇ、Ｃｄ、Ｃｕ、Ｎｉ（Ｍｏ、Ｐｂ、Ｚｎ、Ｂｉ），变生型Ａｕ矿化的元素组合较简单，只为Ａｕ、Ｈｇ、Ａｓ或Ａｕ、Ｃｕ。上述地球化学特征不仅可以有效地区分矿化类型，而且可以作为地球化学找矿和评价的指标  相似文献   

18.
《Applied Geochemistry》2006,21(3):492-514
Geochemical mapping of northern Honshu in the Northeast Japan Arc was carried out using stream sediments at a sampling density of one sample per 130 km2. More than 50 elements were determined in 395 stream sediment samples (<0.18 mm fraction). In geochemical maps, areas with especially low concentrations of large ion lithophile elements (LILE), Be and Li widely overlap with the distribution of Quaternary volcanic rocks along the volcanic front. The areas rich in mafic elements are associated with mafic rocks in many cases. On a regional scale, Ni, Cr and Cu contents are higher in the eastern Paleozoic–Mesozoic basement zone, Pb and Tl tend to be higher on the western zones, and Zn and Cd are high in the western back-arc zone. The areas especially rich in Cu, Zn, Cd, Pb, Bi and Tl coincide with the distribution of large mineral deposits. High concentrations related to Kuroko, hydrothermal-vein, and skarn-type deposits are recognized. High values of As and Sb are related to active geothermal areas near volcanoes and ore deposits. Chemical variations of K, Ce, Th and Sn in the stream sediments are concordant with chemical variations in major rocks. The median and mean concentrations for the stream sediments in northern Honshu, showing arc signatures, are lower in Rb, Cs, Th, Li, K, Be, Ta, LREE, Ni, Hg and Sn, and higher in Sc, Ca and Cd relative to the whole area of Japan, largely because of the contribution of Cenozoic island-arc volcanic rocks that are generally poor in incompatible elements. The averaged chemical compositions of the stream sediments for the geologic zones show systematic variations of many elements. The contrasting variations of LREE and Th contents, which are lower in the zones of Cenozoic rocks relative to the zones of pre-Neogene basements, reflect the regional variations in the main rocks, and also reflect the change of geological settings of the studied area from the continental margin to an island arc during the Cenozoic.  相似文献   

19.
超高压变质岩提供了研究大陆俯冲隧道中岩石的变形机制和流变差异性的窗口。文章使用电子背散射衍射技术分析了大别山超高压变质带的榴辉岩和长英质片麻岩的显微构造。榴辉岩中的石榴子石基本呈无序分布,绿辉石发育较强烈的晶格优选定向,[001]轴的极密平行或近平行于拉伸线理,(100)面的法线近垂直于面理,退变榴辉岩中角闪石的(100)[001]组构可能继承了绿辉石的晶格优选定向。退变榴辉岩和长英质片麻岩中的石英记录了(0001)低温底面滑移和{1010}中温 柱面滑移,反映了超高压变质岩折返到中地壳的韧性变形;而斜长石的(001)<110>和(010)[100]组构形成于折返到下地壳的角闪岩相变质条件（>600℃）。根据主要矿物的流变律计算了俯冲与折返过程中无水矿物的有效黏度变化。俯冲过程中,钠长石=硬玉+石英的分解反应以及石英-柯石英相变导致长英质片麻岩的有效黏度和密度都显著增高,有利于陆壳深俯冲。但是折返过程中由于温度较高,这两个反应带来的有效黏度变化较小。>80 km深度,石榴子石的流变强度>硬玉>绿辉 石>柯石英,俯冲上地壳的流变由柯石英和硬玉控制,下地壳的流变由绿辉石和石榴子石控制。超高压变质岩流变强度的差异有助于上—下地壳力学解耦,使相对低密度、低黏度的上地壳物质在俯冲隧道内快速折返。  相似文献   

20.
In the northwestern Serre, a near continuous section through the Palaeozoic lower continental crust crops out. This paper deals with granulite facies metasedimentary rocks of this section. Fifty-five chemical analyses (major elements, Sc, V, Cr, Co, Ni, Rb, Sr, Y, Nb and Ba) are given. Chemical data indicate that the original sedimentary sequence was made up of arenites in the lower part and mainly Fe and Mg rich pelites in the upper part. The metapelites experienced anatexis and removal of some melt, whereas the meta-arenites suffered only incipient anatexis and became dehydration granulites. The various rock types display an overall relatively low K/Rb ratio due mainly to the stability of K- and Rb-retaining phases. A weighted mean composition of the fragment of the lower continental crust outcropping in the Serre has been established by integrating the chemical data for metasediments (55 analyses), metabasites (92 analyses) and dioritic and tonalitic gneisses (10 analyses) with the field data. This composition compared to the compositions of the bulk continental crust and of other sections considered as belonging to the lower crust, confirms the hypothesis that (1) the lower crust is heterogeneous, and (2) it is more basic than the continental crust.  相似文献   

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